Rolling bearing

ABSTRACT

A rolling bearing made of stainless steel is provided which in use conditions where hydrofluoric acid or fluorine grease is present, can prevent boundary lubrication due to invasion of metal corrosion powder into rolling surface of the bearing, and rapid advance of internal wear and which is highly durable and has a long life even if used in metal corrosive environments. A deep-groove rolling bearing is proposed which comprises bearing rings (inner ring and outer ring), rolling elements and a retainer all made of stainless steel, and solidified lubricating oil sealed in the rolling bearing. The solidified lubricating oil is made by heat curing a mixture of lubricating oil or grease containing perfluoropolyether and a thermoplastic resin. Contact seals made of fluorine rubber seal the solidified lubricating oil. Even if metal abrasion powder is produced by effect of perfluoropolyether or hydrofluoric acid, it will be held embedded in the solidified lubricating oil, so that abnormal wear will not occur.

BACKGROUND OF THE INVENTION

The present invention relates to rolling bearings, particularly torolling bearings suitable for use with a cleaning/chemical polishingapparatus used in environments in which they are brought into contactwith corrosive fluoride compounds for cleaning and chemical polishing.

Generally, in e.g. wafer cleaning and chemical polishing steps insemiconductor manufacturing facilities, corrosive fluoride compoundssuch as hydrofluoric acid are used as cleaning agents or surfacetreatment agents. For example, in a Chemical Mechanical Polishing (CMP)apparatus, wafer surfaces are cleaned and polished by a rotary brushwhile showering hydrofluoric acid on wafers.

Rolling bearings used in or with the carrying unit and rotary brush ofsuch a cleaning apparatus are made of a corrosion-resistant material.For example, bearing rings, rolling elements and retainers made ofstainless steel are adopted (JP patent publication 2003-139147A).

On the other hand, in automotive electric accessories, for whichimproved heat resistance is required, fluorine lubricating grease issealed in rolling bearings used in such accessories and contact sealsmade of fluorine rubber for sealing the gap between inner and outerrings are adopted (JP patent publication 2004-150477A).

The above-mentioned heat-resistant fluorine lubricating grease is madeby mixing perfluoropolyether oil with a thickening agent formed offluorine resin particles such as polytetrafluoroethylene (hereinafterPTFE) for thickening.

Such fluorine lubricating greases are stable to various kinds ofsolvents. Thus they are suitable as lubricating greases for rollingbearings used in a cleaning apparatus.

But if such fluorine lubricating grease is sealed in conventionalrolling bearings having bearing rings, rolling elements and a retainerthat are made of stainless steel, perfluoropolyether (PFPE) mixedtherein as a base oil reacts with stainless steel of the metal materialin a boundary lubrication state, so that the main chain of PFPE is cutat positions of the ether groups and a fluoride (RF-COF) having an acylgroup is produced. The fluoride is a kind of Lewis acid having extremelyhigh corrosivity and acts on PFPE, which is a base oil, as adecomposition catalyst, so that the fluorine lubricating grease will berapidly decomposed and flow away, thus causing metal abrasion of therolling bearing.

By such corrosion mechanism, rolling bearings made of stainless steeland having fluorine lubricating grease sealed therein tend to reach theend of life in a short time with rapid progression of internal wear.

Also, even rolling bearings made of stainless steel and not containingfluorine lubricating grease corrode and produce metallic abrasion powderin use conditions in which they are exposed to hydrofluoric acid as acleaning or polishing agent. The metal abrasion powder infiltrates intorolling portions of the bearing, causing boundary lubrication and thusabnormal bearing wear.

An object of the present invention is to provide a rolling bearing madeof stainless steel which in highly corrosive use conditions wherehydrofluoric acid or fluorine grease is present, can suppress boundarylubrication resulting from infiltration of metal corrosion powder intorolling surface of the bearing and rapid progression of internal wear,and which is highly durable and has a long life even if used in metalcorrosive environments.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a rolling bearingwhich comprises bearing rings, rolling elements and a retainer, whichare all made of stainless steel, a solidified lubricating oil sealed inthe rolling bearing, the solidified lubricating oil being obtained byheat-curing a mixture of lubricating oil or grease and a thermoplasticresin or a thermosetting resin, and contact seals made of fluorinerubber for sealing the solidified lubricating oil.

With the rolling bearing of this invention, because the gap between theinner ring and the outer ring is sealed by contact seals made offluorine rubber, the seals will not be affected by contact with a metalcorrosive agent such as hydrofluoric acid, thus preventing itsinfiltration reliably.

Also, because a solidified lubricating oil obtained by heat-curing amixture of lubricating oil or grease and a thermoplastic resin or athermosetting resin is sealed in the rolling bearing, even if thelubricating oil or grease contains perfluoropolyether and a corrosivefluoride produced in the rolling bearing in boundary lubrication causescorrosion of metal and produces abrasion powder, the metal abrasionpowder will be held as if embedded in the heat cured solidifiedlubricating oil. This prevents it from infiltrating onto the rollingsurfaces (the surfaces of the rolling elements and the surfaces of theinner and outer rings which the rolling elements contact), so thatabnormal wear will not occur.

Also, even if perfluoropolyether (PFPE) reacts with a metal and a metalfluoride is produced, only part of the lubricating oil that has oozedout onto the surface of the solidified lubricating oil may bedecomposed, but the lubricating oil held inside will not be decomposed.Therefore, the rolling surfaces will be kept supplied with normallubricating oil and the solidified lubricating oil will continue tofunction normally.

As described above, because the entire lubricating grease retained inthe solidified lubricating oil is never decomposed at one time or flowsaway, the rolling bearing of this invention can work as a corrosionresistant rolling bearing having a longer life than conventionalbearings of this kind even if used in metal corrosive environments.

In order to provide such a long-life rolling bearing, it is preferablethat the rolling elements used are made of a ceramic material, which hasexcellent corrosion resistance, because a low amount of metalliccorrosion powder produced urges a longer life.

Also, the rolling bearing of such a structure is extremely suitable foruse with a cleaning/chemical polishing apparatus in which they areexposed to a corrosive fluoride such as hydrofluoric acid.

According to the present invention, because a solidified lubricating oilobtained by heat-curing a mixture of lubricating oil or grease and athermoplastic resin or thermosetting resin is sealed in the bearing bycontact seals made of fluorine rubber, if the rolling bearing made ofstainless steel is used in corrosive use conditions where hydrofluoricacid or fluorine grease is present, metallic corrosion powder will notinfiltrate into the rolling surfaces and normal lubricating oil will bekept supplied without being decomposed, thus preventing abnormal wear orrapid progression of internal wear.

Due to these advantages, the rolling bearing of this invention can workas a long life rolling bearing even if used with a cleaning/chemicalpolishing apparatus using corrosive fluorides.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and objects of the present invention will become apparentfrom the following description made with reference to the accompanyingdrawings, in which:

FIG. 1 is a plan view of the rolling bearing according to this inventionwith its seals removed; and

FIG. 2 is an enlarged sectional view of a portion of the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will be described below withreference to the accompanying drawings.

As shown in FIGS. 1 and 2, the embodiment is a deep-groove ball bearinghaving solidified lubricating oil 1 applied to both axial sides of aretainer 2 and heat-cured. Particularly, it comprises an inner ring 3and an outer ring 4 as bearing rings, rolling elements 5 and theretainer 2, which are all made of stainless steel, and contains thesolidified lubricating oil 1 made by heat-curing a mixture of alubricating grease and a thermoplastic resin and sealed by contact seals7 made of fluorine rubber.

The materials of the component parts are described below in detail. Thesolidified lubricating oil 1 is a semi-solid lubricative compositionobtained by mixing a lubricating oil or grease with resin powder. It isapplied to both axial sides of the retainer 2 (that is, both sides ofportions for spacing the rolling elements 5 at equal distances, shown at1 a and 1 b in FIG. 2) and heat-cured so as to be held in position.

The lubricant contained in the solidified lubricating oil used in thisinvention may be lubricating oil or lubricating grease. As thelubricating oil, mineral oil, synthesized hydrocarbon oil, polyalkyleneglycol oil, diester oil, polyolester oil, phosphate ester oil,polyphenyl ether oil, silicone oil, perfluoropolyether oil, etc. may beused. Any known lubricating oil may be used.

The perfluoropolyether oil is a compound obtained by substitutingfluorine atoms for hydrogen atoms of aliphatic hydrocarbon polyether.Commercially available ones include Fomblin Y (made by Montedison),Krytox (made by DuPont), Barrierta J oil (made by Kluber), Fomblin Z(made by Montedison), Fomblin M (made by Montedison) and Demnum (made byDaikin).

The lubricating grease may be one obtained by thickening base oil by useof a soap or non-soap thickening agent. The kinds of the base oil andthickening agent are not particularly limited. Combinations ofthickening agent and base oil include lithium soap-diester grease,lithium soap-mineral oil grease, potassium soap-mineral oil grease,aluminum soap-mineral oil grease, lithium soap-diester mineral oilgrease, non-soap thickening agent—diester oil grease, non-soapthickening agent—mineral oil grease, non-soap thickening agent—polyolester oil grease, lithium soap-polyol ester oil grease, and lithiumsoap-silicone oil grease. Also, a grease using perfluoropolyether oil asbase oil and fluorine resin particles as a thickening agent may be used.

Greases or oils obtained by adding a metallic soap or non-soap (fluorineresin particles, diurea, polyurea, etc.) thickening agent to thelubricating oils described above as a base oil to adjust the viscositymay be used after adding an extreme pressure agent or other additives.

As resin material powder used to solidify a lubricating oil to obtainthe solidified lubricating oil of the present invention, thermoplasticresin powder such as ultrahigh molecular weight polyolefin resin may beused.

As the thermoplastic resin powder, powder of any known thermoplasticresin may be used. Such thermoplastic resins include ultrahigh molecularweight polyolefin powder, polyamide resin (nylon), polyacetal resin,methylmetaacryl resin, acryl-styrene copolymer resin, polystyrene, ABSresin, vinyl chloride resin, polyvinylidene fluoride, polycarbonate,fluoride resin, acetate cellulose, and celluloid.

The ultrahigh molecular weight polyolefin resin powder may be a powderof ultrahigh molecular weight polyethylene, ultrahigh molecular weightpolypropylene, ultrahigh molecular weight polybutene or a copolymerthereof. The molecular weight of each powder should be 1×10⁶ to 5×10⁶ interms of average molecular weight as measured by viscosity method.Polyolefines in such a molecular weight range are superior to lowmolecular weight polyolefines in rigidity and oil retainability andhardly flow even when heated to high temperature. Also, as for averageparticle size of thermoplastic resin powder, favorable results have beenobtained when 10 to 30 μm powder was adopted.

The content of thermoplastic resin in the solidified lubricating oil ispreferably 95 to 1 weight %, though depending on the desired oilseparation, toughness, and hardness of the composition. Therefore, thehigher the content of thermoplastic resin such as ultrahigh molecularweight polyolefine, the harder the gel after dispersed and retained atpredetermined temperature becomes.

As the resin for thermosetting resin powder used as a material of thesolidified lubricating oil used in this invention, phenol resin, urearesin, melamine resin, urethane resin, polyester resin, diarylphthalateresin, and epoxy resin may be used.

As the thermosetting resin, its not-yet-cured material powder is usedand mixed with a lubricating oil or grease. The mixing ratio is notlimited, but they are mixed together in the range of e.g. 5 to 95 weight%. The abovesaid thermosetting resin powder or thermoplastic resinpowder is mixed with a lubricating oil or grease into a uniform paste,semi-solid state with oil not separated, and the mixture is spot-packedin the bearing, or full-packed in the entire space in the bearing. Inspot packing, it is preferable to apply to at least both sides of theportions of the retainer for keeping the spaces between the rollingelements, hold them wrapped and then heat.

The contact seal 7 is an annular elastic member having a core 8 coveredwith fluorine rubber. It is mounted in a mounting groove 9 of the outerring 4 with its seal lip, which is a part of the elastic member and hasan inclined inner periphery, in slidal contact with the sealing contactsurface of the inner ring 3.

As a fluorine rubber used for the contact seal, vulcanizable fluorinerubber containing tetrafluoroethylene-propylene 2-unit copolymer,vulcanizable fluorine rubber containing vinylidenefluoride-tetrafluoroethylene-propylene 3-unit copolymer, or vulcanizablefluorine rubber containing tetrafluoroethylene-perfluoroalkylvinylether3-unit copolymer may be used.

As tetrafluoroethylene-propylene 2-unit copolymer, AFLAS 150 series andAFLAS 100 series of Asahi Glass Co., Ltd. are commercially available.

Vinylidene fluoride-tetrafluoroethylene-propylene 3-unit copolymer iscommercially available in the name of BRE LJ-298005 made by Sumitomo 3MLimited, AFLAS SP and AFLAS MZ201 of Asahi Glass Co., Ltd.

As tetrafluoroethylene-perfluoroalkylvinylether 3-unit copolymer, Kalrezmade by Du Pont can be cited.

Such fluorine polymers described above are vulcanized by use of avulcanizing assistant such as triarylisocyanurate, and triarylcyanurateand organic peroxide such as α,α-bis-t-butylperoxy-diisopropylbenzene,2,5-dimethyl-2,5-di-t-butylperoxy hexane, and exhibit rubber elasticity.

Also, before vulcanizing the abovesaid fluorine polymers, inorganicfillers such as carbon black, silica, silicic acid, and diatomaceousearth, metallic oxides such as zinc oxide and magnesium oxide, ageresistors such as octylated diphenyl amine and N-phenyl-1-naphtyl amineor other additives may be added as necessary.

EXAMPLE 1 OF THE INVENTION

Deep-groove ball bearings (inner diameter: 10 mm, outer diameter: 26 mm,width: 8 mm) with contact seals as shown in FIGS. 1 and 2 were made asfollows: That is, the inner and outer rings were made of stainless steel(SUS440C), the rolling elements were made of silicate nitride ceramics(Si₃N₄), and the retainer was made of stainless steel (SUS304). For thesolidified lubricating oil, lithium-mineral oil grease containingpolyethylene resin powder was used. General-purpose Polylube LP03 madeby NTN and having a softening temperature of about 80° C. and a curingtemperature of about 130° C. was spot packed on both axial sides of theretainer in the bearing, and heat cured. Then, the gap between the outerring and the inner ring was sealed by contact seals of fluorine rubber.The rolling bearings of this invention were obtained by doing so. Therolling bearings were subjected to the following durability tests underthe conditions corresponding to the actual use conditions.

(Durability Test)

Load Fa=10N (preload) was applied to the deep-groove ball bearing withthe contact seals while showering hydrofluoric acid against them. Therotation speed was increased from 0 to 1000 rpm in 30 seconds and thendecreased to zero and stopped in another 30 seconds. Assuming that thisstep represents one cycle, it was repeated 500000 cycles to determinethe life of the bearing.

For judgment of life, the degree of wear inside the bearing wasevaluated in terms of relative widening of the radial inside gap. Thebearing was judged to have ended its life when the radial inside gapwidened to 10 times the initial gap. The test was continued up to 500000cycles if necessary. The results are shown in Table 1.

In addition to the radial inside gap, the bearings were evaluated forcorrosion of the inner and outer rings, corrosion of the rollingelements, sealing by the contact seals, and deterioration of grease infour grades, namely best ⊚, good ◯, passable Δ, and impassable ×. Theevaluation results are shown in Table 1.

TABLE 1 Test item Corrosion of Corrosion Radial inner/outer of rollingSealing Deterioration No. inside gap ring element by seal of lubricantExample ⊚ ◯ ◯ ◯ ◯ of the invention 1 Comparative X X X X X Example 1Comparative X X X X X Example 2 Comparative X X X ◯ X Example 3Comparative Δ X ◯ ◯ X Example 4

COMPARATIVE EXAMPLE 1

A deep-groove ball bearing with non-contact seal made of soft steel(SPCC) (inner diameter: 10 mm, outer diameter: 26 mm, width: 8 mm) wasmade in the following manner. The inner ring, outer ring and rollingelements were made of bearing steel (SUJ2) and the retainer was made ofsoft steel (SPCC), and mineral oil grease (lithium soap used asthickening agent) was sealed as a lubricant. The rolling bearingobtained was tested for durability under the same conditions asdescribed above. The results are shown in Table 1.

COMPARATIVE EXAMPLE 2

A deep-groove ball bearing with contact seals made of nitrile rubber(inner diameter: 10 mm, outer diameter: 26 mm, width: 8 mm) was made inthe following manner. The inner ring, outer ring and rolling elementswere made of bearing steel (SUJ2) and the retainer was made of softsteel (SPCC), and mineral oil grease (lithium soap used as thickeningagent) was sealed as a lubricant. The rolling bearing obtained wastested for durability under the same conditions as described above. Theresults are shown in Table 1.

COMPARATIVE EXAMPLE 3

A deep-groove ball bearing with contact seals made of fluorine rubber(inner diameter: 10 mm, outer diameter: 26 mm, width: 8 mm) made in thefollowing manner. The inner ring, outer ring and rolling elements weremade of stainless steel (SUS440C) and the retainer was made of stainlesssteel (SUS304), and fluorine grease (PTFE powder used as thickeningagent) was sealed as a lubricant. The rolling bearing obtained wastested for durability under the same conditions as described above. Theresults are shown in Table 1.

COMPARATIVE EXAMPLE 4

A deep-groove ball bearing with contact seals made of fluorine rubber(inner diameter: 10 mm, outer diameter: 26 mm, width: 8 mm) made in thefollowing manner. The inner ring and outer ring were made of stainlesssteel (SUS440C) and the rolling elements were made of a ceramic material(Si₃N₄) and the retainer was made of stainless steel (SUS304), andfluorine grease (PTFE powder used as thickening agent) was sealed as alubricant. The rolling bearing obtained was tested for durability underthe same conditions as described above. The results are shown in Table1.

As clearly seen from Table 1, as for Comparative Example 1, hydrofluoricacid invaded into the bearing and the radial inside gap widened tentimes or over and the evaluation was impassable in all other points.

As for Comparative Example 2, the contact seal made of nitrile rubberwas corroded by hydrofluoric acid, which invaded into the bearing, sothat the radial inside gap widened ten times or over.

As for Comparative Example 3, the time required for the widening of theradial inside gap up to ten times was longer in comparison with theComparative Examples 1 and 2, but corrosion and wear of the inner ring,outer ring, races, and rolling elements were observed.

As for Comparative Example 4, corrosion and wear were observed on theraces of the inner ring and the outer ring. Such wear was not observedon the rolling elements, but metal powder produced by abrasion got intothe rolling surface, thus making the rolling elements impossible toturn.

On the other hand, in Example 1, little increase of the radial insidegap was observed even after 500000 cycles in the durability test andgood rotation condition continued.

1. A rolling bearing comprising bearing rings, rolling elements and aretainer, which are all made of stainless steel, a solidifiedlubricating oil sealed in the rolling bearing, said solidifiedlubricating oil being made by heat-curing a mixture of lubricating oilor lubricating grease and a thermoplastic resin or a thermosettingresin, and contact seals made of fluorine rubber for sealing saidsolidified lubricating oil in the rolling bearing.
 2. The rollingbearing of claim 1 wherein said lubricating oil or lubricating greasecontains perfluoropolyether.
 3. The rolling bearing of claim 1 whereinsaid rolling elements are made of a ceramic material.
 4. The rollingbearing of claim 2 wherein said rolling elements are made of a ceramicmaterial.
 5. The rolling bearing of claim 1 which is for use with acleaning/chemical polishing apparatus and is brought into contact with acorrosive fluoride.
 6. The rolling bearing of claim 2 which is for usewith a cleaning/chemical polishing apparatus and is brought into contactwith a corrosive fluoride.
 7. The rolling bearing of claim 3 which isfor use with a cleaning/chemical polishing apparatus and is brought intocontact with a corrosive fluoride.
 8. The rolling bearing of claim 4which is for use with a cleaning/chemical polishing apparatus and isbrought into contact with a corrosive fluoride.
 9. The rolling bearingof claim 5 wherein said corrosive fluoride is hydrofluoric acid.
 10. Therolling bearing of claim 6 wherein said corrosive fluoride ishydrofluoric acid.
 11. The rolling bearing of claim 7 wherein saidcorrosive fluoride is hydrofluoric acid.
 12. The rolling bearing ofclaim 8 wherein said corrosive fluoride is hydrofluoric acid.